A heat exchanger is used as one element of a refrigeration cycle. The heat exchanger is an essential part for changing a temperature of a working fluid in the refrigeration cycle to a desired temperature. Various heat exchangers exist. In particular, microchannel heat exchangers have excellent performance, which is becoming more and more known. The microchannel heat exchangers are being developed for practical application.
Those microchannel heat exchangers include a stacked microchannel heat exchanger. This stacked microchannel heat exchanger is configured as follows, for example. A stack is formed by alternately stacking heat transfer plates having surfaces in which minute high-temperature channels are formed and heat transfer plates having surfaces in which minute low-temperature channels are formed. Pressure-resistant metal plates are disposed on an upper surface and a bottom surface of the stack, and pressed and heated in a vacuum state. In this manner, the heat transfer plates and the metal plates are diffusion-welded and integrated with one another (e.g., Non-Patent Literature 1).
Structural characteristics of the stacked microchannel heat exchanger as compared to a plate-type heat exchanger can include capability of forming a larger number of channels in each layer, capability of forming short channels, and the like. With this, the stacked microchannel heat exchanger can be downsized in comparison with the plate-type heat exchanger.
Further, the stacked microchannel heat exchanger has more excellent points also in view of heat transfer property, coolant filling amount, higher pressure-resistance, and heat-resistance in comparison with conventional heat exchangers. For example, the coefficient of overall heat transmission between working fluids via a heat transfer surface (plate) is large, the channel shape loss is low, the channel area can be reduced if the flow loss is equal to that of the plate-type heat exchanger, the pressure loss of compressed working fluids can be reduced, the amount of working fluid filling the refrigeration cycle can be reduced due to the reduced volume of the entire heat exchanger, etc.
The outlet and inlet of the stacked microchannel heat exchanger which working fluids exits and enters are provided with temperature sensors. The temperature sensors are provided for the purpose of calculating a quantity of heat exchanged in the heat exchanger on the basis of temperatures measured by the temperature sensors and controlling a flowing-out working fluid to a desired temperature.
For accomplishing this purpose, the temperature sensors need to be capable of correctly measuring temperatures of working fluids. For example, in a case where heat is exchanged between two working fluids, the heat exchange capability (amount of heat transferred) of the heat exchanger can be calculated on the basis of a temperature difference between a flowing-in working fluid and a flowing-out working fluid in accordance with the following expression.
      Q    ⁡          (                        [                      J            ⁢                          /                        ⁢            s                    ]                =                  [          W          ]                    )        =                    c                  p          ,          1                    ⁢                          ⁢              (                  [                      J            ⁢                          /                        ⁢            kgK                    ]                )            ×              G        1            ⁢                          ⁢              (                  [                      kg            ⁢                          /                        ⁢            s                    ]                )            ×              (                              T                          Low              ,              out                                -                      T                          Low              ,              in                                      )            ⁢                          ⁢              (                  [          K          ]                )              =                  c                  p          ,          h                    ⁢                          ⁢              (                  [                      J            ⁢                          /                        ⁢            kgK                    ]                )            ×              G        h            ⁢                          ⁢              (                  [                      kg            ⁢                          /                        ⁢            s                    ]                )            ×              (                              T                          High              ,              in                                -                      T                          High              ,              out                                      )            ⁢                          ⁢              (                  [          K          ]                )            
Q: amount of heat transferred [J/s]=[W]
cp, l: specific heat [J/kgK] of low-temperature working fluid
cp, h: specific heat [J/kgK] of high-temperature working fluid
Gl: mass flow rate [kg/s] of low-temperature working fluid
Gh: mass flow rate [kg/s] of high-temperature working fluid
(TLow, out−TLow, in): (temperature difference [K] between heat-exchanger outlet temperature of low-temperature working fluid and inlet temperature of low-temperature working fluid)
(THigh, in−THigh, out): (temperature difference [K] between heat-exchanger inlet temperature of high-temperature working fluid and outlet temperature of low-temperature working fluid)
Further, with a water heater or the like, it is necessary to correctly measure a temperature of a working fluid flowing through an outlet of a microchannel heat exchanger for checking whether or not the working fluid has reached a desired temperature. Further, it is necessary to correctly measure a temperature of the working fluid flowing through an inlet of the microchannel heat exchanger for checking whether or not it is necessary to heat a working fluid flowing out of a hot water tank and also for deriving a quantity of heat required for heating the working fluid to a desired temperature.
For measuring the temperatures of the working fluids flowing through the outlet and inlet of the stacked microchannel heat exchanger, temperature sensors such as thermocouples are used.
Thermoelectromotive force measured at a sensing point of each temperature sensor is transmitted to a thermoelectromotive force-to-temperature conversion circuit via a thermocouple wire continuous with the sensing point. In many cases, the temperature sensor is fixed to an outer surface of a pipe, which is attached to each of the inlet and the outlet for the working fluid of the heat exchanger, by soldering. In this case, the sensing point of the temperature sensor is not in direct contact with the working fluid, and hence it is impossible to correctly measure the temperature of the working fluid.
Therefore, the measured temperature has an error 1 due to heat conduction of the metal forming the heat exchanger, an error 2 due to a temperature difference between a temperature of a position at which the temperature sensor is attached and an actual temperature of the working fluid flowing through the outlet/inlet, an error 3 due to a temperature difference between a temperature of the working fluid flowing near a center of the pipe and a temperature of the working fluid flowing near a wall surface of the pipe due to a temperature boundary layer of the working fluid flowing through outlet/inlet pipe connected to the outlet/inlet, a measurement error 4 of a measurement method of the temperature sensor, and the like.